Establishing Water Quality-based Effluent Limitations in ...Receiving Water Characteristics (Part II) For steady-state modeling under critical conditions: If the receiving water concentration

WQBELs Part III

WQBELs Part III-1

Determine the Need for WQBELs

NPDES Web-based Training

Establishing Water Quality-based Effluent Limitations in NPDES Permits:

Part III—Determine the Need for WQBELs

WQBELs Part III-2WQBELs Part III-2WQBELs Part III-2WQBELs Part III-2WQBELs Part III-2

Today’s Speakers

David Hair

Environmental Engineer

US Environmental Protection Agency

Washington, DC

Greg Currey

Environmental Engineer

Tetra Tech, Incorporated

Fairfax, Virginia

WQBELs Part III

WQBELs Part III-2



WQBELs Part III-3

Establishing WQBELs in NPDES Permits

Part IV: Calculate Chemical-specific WQBELs

and Determine Final Limitations

Part III: Determine the Need for WQBELs

Part II: Characterize the Effluent and Receiving Water

Part I: Identify Applicable Water Quality Standards

WQBELs Part III-4

Part I Review: Relationship Between WQS and Effluent Limitations

Recall from Part I (Identify Applicable WQS):

– Water quality standards apply throughout

the waterbody (or segment of a

waterbody) as defined by the state,

territory, or tribe

– Effluent limitations apply at the

compliance point established in the

permit (generally “end of pipe”)

WQBELs Part III

WQBELs Part III-3



WQBELs Part III-5

Part I Review: Relationship Between WQS and Effluent Limitations

Water Quality Criteria

– Magnitude

– Duration

– Frequency

Effluent Limitations

– Magnitude

– Averaging

Period

1-5

Permit writers calculate end-of-pipe water quality-based effluent limitations where necessary to ensure that water quality standards are attained in the receiving water.

WQBELs Part III-6

Part I Review: WQS Implementation Procedures

Water quality standards and their implementing procedures (including NPDES requirements) specify methods for determining the need for WQBELs and for calculating WQBELs that ensure that standards are attained.

Where can these methods be found?

– EPA’s Technical Support Document

– State regulations

– State water quality management plans

– State guidance

– past practices

– We never thought about this before!

WQBELs Part III

WQBELs Part III-4



WQBELs Part III-7

Part II Review: Identify Pollutants of Concern

Recall from Part II (Characterize Effluent and Receiving Water) that pollutants of concern are pollutants:

– With an applicable technology-based effluent limitation (TBEL)

– With a wasteload allocation (WLA) from a total maximum daily load (TMDL)

– Identified as needing WQBELs or monitoring in the previous permit

– Identified as present in the effluent through monitoring

– Otherwise expected to be present in the discharge

Point

Source #3

WQBELs Part III-8

Also recall from Part II (Characterize Effluent and Receiving Water) that we need to:

Determine whether water quality standards permit dilution allowances or mixing zones

Determine critical conditions (e.g., critical stream flow)

Determine type of mixing under critical conditions

rapid and complete mixing

incomplete mixing

Determine dilution allowance or regulatory mixing zone size for calculations

Part II Review: Determine the AllowableDilution or Mixing Zone in the Receiving Water

WQBELs Part III

WQBELs Part III-5



WQBELs Part III-9

Establishing WQBELs in NPDES Permits

Part IV: Calculate Chemical-specific WQBELs

and Determine Final Limitations

Part III: Determine the Need for WQBELs

Part II: Characterize the Effluent and Receiving Water

Part I: Identify Applicable Water Quality Standards

WQBELs Part III-10

Part III: Determining the Need for Chemical-specific WQBELs

When must a permit writer establish effluent

limitations using water quality criteria?

Answer: Limitations must be established in

permits to control all pollutants or pollutant

parameters that are or may be discharged at a

level that will cause, have the reasonable potential

to cause, or contribute to an excursion above any

state water quality standard [40 CFR

122.44(d)(1)(i)].

WQBELs Part III

WQBELs Part III-6



WQBELs Part III-11

Is There Reasonable Potential?

Effluent and

Receiving Water

Characteristics

(Part II)

Limitations must be established in permits

to control all pollutants or pollutant

parameters that are or may be discharged

at a level that will cause, have the

reasonable potential to cause, or contribute

to an excursion above any state water

quality standard.

Water Quality Model (steady-state or

dynamic)

Receiving Water Concentration

WQBELs Part III-12

Steady-State Modeling

Predicts the impact of the effluent on

the receiving water for a single set of

conditions

Can be used in both rapid and complete

mixing and incomplete mixing situations

Generally assumes that the single set of

conditions are the critical conditions for

flow, pollutant concentrations, and

environmental effects

WQBELs Part III

WQBELs Part III-7



WQBELs Part III-13

Dynamic Modeling

Accounts for variability of model

inputs

Projects probability distributions

rather than a single value based

on critical conditions

Data intensive and complex

WQBELs Part III-14

Is There Reasonable Potential?

Effluent and

Receiving Water

Characteristics

(Part II)

For steady-state modeling under critical conditions:

If the receiving water concentration exceeds the applicable water quality criterion, then there is reasonable potential and the permit writer mustestablish WQBELs

If the receiving water concentration is equal to or less than the applicable water quality criterion, then there is no reasonable potential and we have not demonstrated a need to establish WQBELs

Water Quality ModelReceiving Water Concentration

WQBELs Part III

WQBELs Part III-8



WQBELs Part III-15

Expected Receiving Water Concentrations for Steady-State, Incomplete Mixing Under Critical Conditions

Steady-State, Incomplete Mix Assessment

Acute Aquatic Life Water Quality Criterion for Pollutant X = 1.0 mg/L

The discharge of Pollutant X from ABC, Inc., would

cause, have the reasonable potential to cause, or

contribute to an excursion of the acute aquatic life

criterion.

The permit writer must calculate WQBELs for

Pollutant X.

WQBELs Part III-16

Expected Receiving Water Concentration for Steady-State, Rapid and Complete Mixing Under Critical Conditions

Mass-Balance Equation: QsCs + QdCd = QrCr

Q = Flow (mgd or cfs)

C = Pollutant concentration (mg/l)

Mass = [Concentration] [Flow]

WQBELs Part III

WQBELs Part III-9



WQBELs Part III-17

Determine the pollutant concentration of Pollutant X (the pollutant of concern) in the water body downstream of the discharge:

Cr = QsCs + QdCdQr

Steady-State Complete Mix Assessment QsCs + QdCd = QrCr

WQBELs Part III-18

Calculating Receiving Water Concentration Under Critical Conditions

Criterion for protection of aquatic life from acute effects from Pollutant X: = 1.0 mg/L

Qs = Critical stream flow (1Q10) for acute criterion = ???????

Qd = Critical effluent flow from discharge flow data = ???????

Qr = Sum of critical stream flow and critical effluent flow = ???????

Cs = Critical upstream pollutant concentration = ???????

Cd = Critical effluent pollutant concentration = ???????

Cr = QsCs + QdCdQr

WQBELs Part III

WQBELs Part III-10



WQBELs Part III-19

Cr = QsCs + QdCdQr

Qs = Critical stream flow from water quality standards


WQBELs Part III-20



Qs = Critical stream flow (1Q10) for acute criterion = 1.2 cfs

Qd = Critical effluent flow from discharge flow data = ???????




Cr = QsCs + QdCdQr

WQBELs Part III

WQBELs Part III-11



WQBELs Part III-21

Cr = QsCs + QdCdQr

Qd = Critical effluent flow from discharge flow data


WQBELs Part III-22




Qd = Critical effluent flow from discharge flow data = 0.31 cfs




Cr = QsCs + QdCdQr

WQBELs Part III

WQBELs Part III-12



WQBELs Part III-23

Cr = QsCs + QdCdQr

Qr = Sum of upstream flow (Qs) and discharge flow (Qd)


WQBELs Part III-24





Qr = Sum of critical stream flow and critical effluent flow = 1.51 cfs



Cr = QsCs + QdCdQr

WQBELs Part III

WQBELs Part III-13



WQBELs Part III-25

Cr = QsCs + QdCdQr

Cs = Critical background (upstream) pollutant

concentration from ambient monitoring data


WQBELs Part III-26






Cs = Critical upstream pollutant concentration = 0.80 mg/L


Cr = QsCs + QdCdQr

WQBELs Part III

WQBELs Part III-14



WQBELs Part III-27

Cr = QsCs + QdCdQr

Cd = Critical effluent pollutant concentration


WQBELs Part III-28

Examine data for ABC, Incorporated

– Number of samples (N) = 6

– Concentrations of Pollutant X:Cd(1) = 1.2 mg/L Cd(2) = 0.92 mg/L

Cd(3) = 0.87 mg/L Cd(4) = 1.3 mg/L

Cd(5) = 0.74 mg/L Cd(6) = 1.0 mg/L

– Maximum Observed Value

of Effluent Concentration = 1.3 mg/L

– Would this Cd represent the

“critical” condition?

Determining a Maximum (Critical) Value for Cd

WQBELs Part III

WQBELs Part III-15



WQBELs Part III-29

Determining a Maximum (Critical) Value for Cd

Answer: Not likely

– Our limited data set does not account for

day-to-day variability in effluent quality

(i.e., the facility probably did not self-

monitor on its worst possible day).

– When determining reasonable potential,

“…..the permitting authority shall use

procedures which account for…..the

variability of the pollutant or pollutant

parameter in the effluent…” [40 CFR

122.44(d)(1)(ii)].

WQBELs Part III-30

Determining a Maximum(Critical) Value for Cd

Follow permitting authority

procedures to determine the critical

value for Cd

– Permitting authority regulation, policy,

or guidance

– EPA’s Technical Support Document for

Water Quality-based Toxics Control

(TSD)

• Uses a statistical analysis that assumes

effluent data follow a lognormal distribution

WQBELs Part III

WQBELs Part III-16



WQBELs Part III-31

Some Key Terms

Lognormal Distribution: the probability distribution of any

random variable whose logarithm is normally distributed

– Relative Frequency: the fraction or ratio of the number of

observations in a category or class to the total number of

observationsR

ela

tive

Fre

qu

en

cy

0 Concentration

Lognormal

Distribution

WQBELs Part III-32

Some Key Terms

Long-term Average (LTA): for a continuous random variable (in our case, pollutant concentration), the value at which the area under the distribution curve to the left of the value equals the area under the distribution curve to the right of the value

Coefficient of Variation (CV): a statistical measure of the relative variation of a distribution or set of data (in our case, pollutant concentrations) calculated as the standard deviation divided by the mean

WQBELs Part III

WQBELs Part III-17



WQBELs Part III-33

Defining a Lognormal Distribution Using LTA and CV

Rela

tive

Fre

qu

en

cy

0

CV

LTA

Concentration

WQBELs Part III-34

Cr = QsCs + QdCdQr

Recall….we want to determine the critical

effluent pollutant concentration (Cd)


WQBELs Part III

WQBELs Part III-18



WQBELs Part III-35

Determining a Critical Value for Cd

Rela

tive

Fre

qu

en

cy

0

CV

LTA

Concentration

Set Critical Value of Cd

at 99th Percentile

Critical Value for Cd on

Tail of Curve

WQBELs Part III-36

Examine data for ABC, Incorporated using

the TSD statistical approach



Cd(3) = 0.87 mg/L Cd(4) = 1.3 mg/L

Cd(5) = 0.74 mg/L Cd(6) = 1.0 mg/L

– CV = 0.6 (EPA recommends a default CV value of

0.6 if there are < 10 data points available)




WQBELs Part III

WQBELs Part III-19



WQBELs Part III-37

Statistics tell us that we can be 99% sure that the largest value of

our 6 measurements of the concentration of Pollutant X will be at

or greater than the 46th percentile of the lognormal distribution of

all effluent pollutant concentrations for ABC, Inc.


Concentration

Re

lati

ve

Fre

qu

en

cy

0

46th percentile

concentration

WQBELs Part III-38

To be 99 percent sure that we have captured the 99th percentile

concentration of Pollutant X (which we will call the critical or

upper-bound concentration), we need the highest concentration

measured from 330 samples of ABC Inc.’s effluent


Concentration

Rela

tive

Fre

qu

en

cy

0

99th percentile

concentration

WQBELs Part III

WQBELs Part III-20



WQBELs Part III-39


Our options:1. Measure the concentration of Pollutant X in

330 separate samples of ABC, Inc.’s effluent

2. Use statistics for the lognormal distribution to find a multiplier that lets us estimate the 99th

percentile (which is what we want to find) from the 46th percentile (which is represented by the highest of our 6 measured concentrations)

For any data set, to estimate the upper bound value, we need to know:– Number of samples collected (N)

– Coefficient of variation (CV)• Use a default of 0.6 if N < 10

WQBELs Part III-40

Reasonable Potential Multiplying Factors

(99% Confidence Level and 99% Probability Basis

Sample

NumberCoefficient of Variation

N 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

1 2.5 6.0 13.2 26.5 48.3 81.4 128.0 190.3 269.9 368.3

2 2.0 4.0 7.4 12.7 20.2 30.3 43.0 58.4 76.6 97.5

3 1.9 3.3 5.6 8.9 13.4 19.0 25.7 33.5 42.3 52.0

4 1.7 2.9 4.7 7.2 10.3 14.2 18.6 23.6 29.1 35.1

5 1.7 2.7 4.2 6.2 8.6 11.5 14.8 18.4 22.4 26.5

6 1.6 2.5 3.8 5.5 7.5 9.8 12.4 15.3 18.3 21.5

7 1.6 2.4 3.6 5.0 6.7 8.7 10.8 13.1 15.6 18.2

8 1.5 2.3 3.3 4.6 6.1 7.8 9.6 11.6 13.6 15.8

9 1.5 2.2 3.2 4.3 5.7 7.1 8.7 10.4 12.2 14.0

10 1.5 2.2 3.0 4.1 5.3 6.6 8.0 9.5 11.0 12.6

11 1.4 2.1 2.9 3.9 5.0 6.2 7.4 8.8 10.1 11.5

12 1.4 2.0 2.8 3.7 4.7 5.8 7.0 8.1 9.4 10.6

13 1.4 2.0 2.7 3.6 4.5 5.5 6.5 7.6 8.7 9.9

14 1.4 2.0 2.6 3.4 4.3 5.2 6.2 7.2 8.2 9.2

15 1.4 1.9 2.6 3.3 4.1 5.0 5.9 6.8 7.7 8.7

16 1.4 1.9 2.5 3.2 4.0 4.8 5.6 6.5 7.3 8.2

17 1.4 1.9 2.5 3.1 3.8 4.6 5.4 6.2 7.0 7.8

18 1.4 1.9 2.4 3.0 3.7 4.4 5.2 5.9 6.7 7.4

19 1.4 1.8 2.4 3.0 3.6 4.3 5.0 5.7 6.4 7.1

20 1.3 1.8 2.3 2.9 3.5 4.2 4.8 5.5 6.1 6.8

3.8

WQBELs Part III

WQBELs Part III-21



WQBELs Part III-41

Examine data for ABC, Incorporated using

the TSD statistical approach



Cd(3) = 0.87 mg/L Cd(4) = 1.3 mg/L

Cd(5) = 0.74 mg/L Cd(6) = 1.0 mg/L

– CV = 0.6 (default value if N < 10)




WQBELs Part III-42


Projected Critical (99th percentile) Value of Cd =

1.3 mg/L x multiplier =

1.3 mg/L x 3.8 = 5.0 mg/L

Cd = 5.0 mg/L

WQBELs Part III

WQBELs Part III-22



WQBELs Part III-43






Cs = Critical upstream pollutant concentration = 0.80 mg/L

Cd = Critical effluent pollutant concentration (projected) = 5.0 mg/L

Cr = QsCs + QdCdQr

WQBELs Part III-44

Cr = (1.2 cfs)(0.80 mg/L) + (0.31 cfs)(5.0 mg/L)

Cr = 1.7 mg/L

(1.2 cfs) + (0.31 cfs)

Expected Receiving Water Concentration(Steady-State, Rapid and Complete Mix Under Critical Conditions)

Cr =QsCs + QdCd

Qr

WQBELs Part III

WQBELs Part III-23



WQBELs Part III-45

Is There Reasonable Potential?(Steady-State, Rapid and Complete Mix Under Critical Conditions)

For ABC, Incorporated:– Projected Cr = 1.7 mg/L > 1.0 mg/L (acute criterion)

– The discharge of Pollutant X from ABC, Incorporated would cause, have the reasonable potential to cause, or contribute to an excursion of the acute aquatic life criterion.

– The permit writer must calculate WQBELs for Pollutant X.

WQBELs Part III-46

In our example, where we considered only the acute aquatic life criterion, we still would need to consider, if available:– chronic aquatic life criterion

– human health criteria

– wildlife criteria

– etc.

What Next?

WQBELs Part III

WQBELs Part III-24



WQBELs Part III-47

What Next? (continued)

Repeat the entire analysis for additional pollutants of concern and additional outfalls

For each pollutant for which we determine there is reasonable potential to exceed anyof the criteria for that pollutant, calculate chemical-specific WQBELs (Part IV)

When there is no reasonable potential– determine whether any existing limitations

should be retained

– consider appropriate monitoring requirements

WQBELs Part III-48

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Documents

Establishing Water Quality-based Effluent Limitations in ...Receiving Water Characteristics (Part II) For steady-state modeling under critical conditions: If the receiving water concentration